Electric transformer



Feb. 22, 1949- w. KRAM ELECTRIC TRANSFORMER 3 Sheets-Sheet 1 Filed April26, 1946 W f. m 7 e Z 4 m V 0 ,4 0 W 4 M M as m \9 8 5 g k m A F4 3/3?Feb, 22, 1949. w, KR M 2,462,106

ELECTRIC TRANSFORMER Filed April 26, 1946 3 Sheets-Sheet 2 Patented Feb.22, 1949 ELECTRIC TRANSFORMER Walter Kram,

London, England, assignor, by

memo assignments, to International Standard Electric Corporation, NewYork, N. Y., a corpcration of Delaware Application April 26, 1946,Serial No. 665,047 In Great Britain April 27, 1945 7 Claims. 1

The present invention relates to improvements in double screenedelectrical transformers. Such transformers have been employed for a longtime for use in alternating current bridges and for coupling highfrequency circuits together, and for similar uses. It has been found,however, that the designs hitherto adopted for these transformers proveinadequate when the frequencies employed extend into the radio ire.-quency range. It is the principal object of this specification,therefore to disclose the main cause for the defects of the existingdesigns and to show how they may be remedied.

The invention will be described with reference to the accompanyingdrawings, in which:

Fig. 1 shows a schematic circuit diagram of a known type of doublescreened transformer;

Figs. 2 and 3 show two test circuits employed in the explanation of thedistinction between the static and dynamic capacitances associated withsuch a transformer;

Fig. 4 shows a perspective view of the winding and screeningarrangements of a double screened transformer;

Figs. 5, '7 and 8 show diagrams illustrating one method of designing adouble screened transformer according to the invention;

Fig. 6 shows a schematic circuit diagram to illustrate the effect of thescreen electromotive forces in a double screened transformer;

Fig. 9 shows equivalent circuits employed in the explanation of theaction of the screen electromotive forces;

Fig. 10 shows a double screened transformer with neutralisingarrangements according to the invention;

Figs. 11 and 12 show another neutralising arrangement according to theinvention;

Fig. 13 shows still another arrangement according to the invention;

Fig. 14 shows a schematic circuit diagram of a screened impedance bridgeof known type;

Fig. 15 shows a simplified circuit diagram of Fig. 14 to illustrate theeffect of the screen electromotive forces;

Fig. 16 shows a transforming arrangement ac cording to the inventionapplicable to the bridge shown in Fig. 14; and

Fig. 1'7 shows a simplified circuit to illustrate the action of Fig. 16.

Referring to Fig. 1, which shows a schematic circuit diagram of adouble-screened transformer of well known type, it will be seen that thetransformer consists of a primary winding 5 coupled inductively to asecondary winding 2.

The secondary winding is substantially completely enclosed in an innermetal screen 3, and this screen is in turn substantially completelyenclosed in an outer metal screen 4 over which the primary winding i iswound. The whole is then enclosed in an outer metal case 5 which isconnected at one point to the screen l. Terminals 6, i for the primarywinding l are located inside the outer screen, and terminals 8, 9 forthe secondary winding are located inside the inner screen. A terminalit) for the inner screen 3 is located inside the outer screen and H isthe terminal for the outer screen. The construction of the transformerwill be explained in fuller detail below with reference to Fig. 4.

When the transformer is connected in a circuit, suitable singleordouble-screened connecting leads are employed to maintain the continuityof the screening from the external circuit components right up to thetransformer terminals.

The principal screening requirements for such a transformer havehitherto been that the direct capacitance between the primary winding land the inner winding 2 or the inner screen 3 should be substantiallyzero, and also that the direct capacitance between the inner winding 2and the outer screen 4 should be substantially zero. This is principallyfor the purpose of ensuring that no alternating electromotive forces orpoten tial differences are produced on the secondary side of thetransformer by currents flowing in the primary winding except theelectromotive force generated in the secondary winding 2 by directmagnetic coupling. It is also for the purpose of definitely fixing andlocalising the capacitances associated with the transformer so thattheir effects can be properly controlled. All this is, of course, wellknown practice.

it is not possible to completely reduce the acovementioned capacitancesto Zero, on account of design and manufacturing limitations, and it hastherefore been the practice to set maximum limits for these capacitanceswhich it is judged will be sufficient to prevent the undesired effectsresulting therefrom from becoming excessive. However, it has been foundthat when an attempt is made to employ these conventional transformersat radio frequencies, they behave as if the above-mentioned directcapacitances were much larger than their measured values. Fuller detailsof this effect will be given presently, but it may be stated here thatan investigation of the trouble has shown that the effect is principallydue to the electromotive forces which are generated in the screens bythe currents in the transformer windings. These electromotive forcesoperate in conjunction with the capacitance between the inner and outerscreens to produce unwanted currents in the circuits connected to thesecondary side of the transformer.

This effect may produce serious errors in the measurements made at radiofrequencies on impedance bridges employing such double screenedtransformers; and particularly in cases where the impedance to bemeasured has one component very small compared with the other, verylarge percentage errors may be produced in the measurement of thesmaller component. Where the transformers are used at very highfrequencies for coupling transmission circuits, for example, the effectmay result in the introduction of excessive crosstalk or noise.

The recognition of the manor cause of this trouble has enabled severalpossible remedies to be applied which form features of the presentinvention.

When determining the values of the direct capacitances associated with adouble-screened transformer, it is usual to short-circuit the primaryand secondary windings so that they can be treated substantially assingle unipotential conductors, and to make the measurements on a directcapacitance bridge, of which there are several well known types. Suchmeasurements are found to produce correct values for these capacitances.When the transformer is con nected in a normal circuit, however,currents flow through the windings, and an alternating flux is developedin the core which is a condition not taken into account in theabove-mentioned measurements of the direct capacitances, which aretherefore not the only factors which affect the screening of thetransformer. Figs. 2 and 3 show schematic circuit diagrams of twomeasuring circuits by which the effect of the screen electromotiveforces may be made evident.

In these figures, i2 is a source of alternating current of impedance Rconnected through an adjustable attenuator [3 of constant characteristicimpedance R. to the double screened transformer which is similar to thatdescribed with reference to Fig. 1. The output side of the transformeris connected to a detector M of conventional type, of input impedancealso R. In Fig. 2 both windings of the transformer are short circuited,and terminal I is connected to the outer screen 5 through a resistancel5 equal to the characteristic impedance R of the attenuator 3. Theoutput side of the attenuator is connected to terminals 5 and H, thelatter terminal being connected to ground. The input terminals of thedetector it are connected to terminals ID and H. The leads fromterminals 6 and I ii should have screens connected to the outer screen5, as indicated. The attenuator I3 is adjusted to obtain a convenientreading on the meter of the detector Hi. Suitable switching means (notshown) of known character is provided to connect the output of I 3direct to the input of I 4, thus cutting out the transformer. Theattenuator I3 is then readjusted to produce the same reading on thedetector meter. The difference between the two readings of attenuator l3then gives the attenuation of the connection formed through the directcapacitance between the primary winding and the inner screen 3. It iseasily shown that this reading should be equal to A decibels where A=10logio (1+1/4w C R (1) in which 0 is the value of the direct capacitancebetween the winding l and the screen 3, R is the characteristicimpedance of the attenuator l3 and w is 21r times the frequency.

The value of 0 obtained from this measurement is found to agreesatisfactorily with the value measured on a direct capacitance bridge.

Fig. 3 differs from Fig. 2 in that neither winding is short circuited,and terminal l is connected direct to the outer screen 5. The resistancei5 is connected between the output terminals 8 and 9. It is assumed thatthe transformer has a 1:1 ratio and is designed for coupling togethertwo impedances each equal to R.

When the test is made with the arrangement of Fig. 3 in the same way ashas been described with reference to Fig. 2, it is found that theattenuation measurement obtained is very much lower than before, whichwould indicate that the transformer is behaving as though the directcapacitance 0 were considerably larger than the measured value.

It will be convenient to call the direct capacitance measured accordingto Fig. 2, or by means of a direct-capacitance bridge the staticcapacitance, and the apparent value derived according to the equation(1) from the measurement according to Fig. 3 the dynamic capacitance.

To give specific examples, it was found that in the case of an equalratio transformer designed for R=600 ohms in which the staticcapacitance was 0.05 F, the dynamic capacitance measured at 50kilocycles per second was about 1.3 ,uuF, or about 26 times as much. Inthe case of an equal ratio transformer designed for R= ohms, in whichthe static capacitance was about 0.1 ,u/LF, the dynamic capacitance wasfound to be about 30 (L111? at the same frequency.

It will be appreciated from what has been explained already that thedynamic capacitance is only an apparent effect, the cause of which isthe spurious electromotive forces developed in the transformer screens,but it serves as a convenient indication of the magnitude of thecoupling produced by the said spurious electromotive forces.

In order to make clear the origin of the dynamic capacitance effect,Figs. 4 and 5 have been prepared. The core of a double screenedtransformer may be either solenoidal, toroidal or of the shell type; inall cases the portion on which the windings are mounted approximateseither to a cylinder or to a toroid of circular section. In theperspective view of Fig. 4, the inner winding 2 is mounted directly onthe central core l6 and is completely surrounded by the innercylindrical or toroidal portion of the screen 3. This is in turncompletely surrounded by the outer cylindrical or toroidal portion 4 ofthe outer screen. The outer winding I is wound over the outer screenportion 4 as indicated, and the whole is enclosed in an outer metal box5 shown diagrammatically as a rectangle enclosing the rest of theassembly. The two screen portions 3 and 4 are split at I! and I8 inlines parallel to the axis of the core i 6 in order to prevent thescreen portions acting as short-circuited turns. An overlap is formedwith suitable insulation (not shown) between the two portions of theoverlap in order to reduce the unavoidable gap in the screen produced bythe split. A wire I9 is attached to a convenient point 2 l of the screen3, a second wire for connection to the corresponding terminal Ill. 20 isattached to a convenient point 22 of the screen 4 and connects it to asingle point on the outer box 5 and thence to the corresponding terminalII. This arrangement is well known, and no details are given of themeans for insulating the screens from one another and from the windings,nor of the mounting and potting arrangements.

Fig. 5 shows a diagram of a section through the two screens takenperpendicular to the core it. The centre of the section is the point I3,and two dotted lines UP and 0Q have been drawn through the gaps in thetwo screens 4 and 3, respectively. It is to be noted that owing to theoverlaps, the effective gaps are at the points indicated at thetermination of the inner and outer laps respectively of the screens aand 3. The angle between 0? and 0Q is taken as Q and the radii from 0 tothe connecting points 2| and 22 make angles ,8 and a, respectively, with(JP, which is the reference radius.

Now it will be understood that although no current flows in eitherscreen when an alternating current flows in either or both of thetransformer windings, yet an electromotive force is developed thereinequal to the electromotive force E generated in a single turn of thesecondary winding. same for both screens if it be assumed that the wholeof the flux is concentrated inside the inner screen 3, which issubstantially the case. Assuming also that the resistance of the screenmaterial is negligible compared with the external cir- P-I' cuitimpedance, and with the outer screen impedance, it can be assumed thatin the case of the screen 4 the difference of potential between pointsof the screen adjacent to and on opposite sides of the line 0? is E. Ifthe point on the clockwise side be taken as the reference point at zeropotential, then the potential at other points increases proportionallyto the clockwise angle measuredfrom 0P up to the value +E immediately onthe anti-clockwise side. Similarly for the inner screen 3, the zeroreference point is on the clockwise side of 0Q and the potentialincreases proportionally with the clockwise angle from 0Q up to thevalue +E immediately on the anticlockwise side of 0Q. These results arebased on the simplifying assumption that the flux distribution in thecore is uniform, and that the screens are both circular and concentrictherewith.

Since discontinuities occur in the screens respectively at P and Q, itwill be necessary to consider the current flow and potentials in thesectors q: and (21r separately.

Let I be the current in the impedance Z of the external circuitconnected between the points 2] and 22. Take any radius DAB in thesector (21r-(p) making a clockwise angle 0, with DP. Then if a and b arethe potentials of A and B respectively with respect to the point 2| Itwill be seen that this is independent of 01 so that the difference ofpotential between the two screens is constant over the sector 21r-q).

This electromotive force will be the Similarly, take anyradius 0CD inthe. sector c making a clockwise angle 62 with HP. .Let .c and d be thepotentials of C andD respectively with respect to the point 2 I. ThenThisis independent of 02 so that the difference of potential between thetwo screens is also'constant over the sector q), and differs from thevalue (2) for the other sector by E.

It will now be assumed that the admittance between the screens in eachsector is made up of a conductance component and a susceptance componentboth of which are proportional to the angle subtended by the sector; inother words the admittance for the sector 21r-qo is and the admittancefor the sector (p is likewise This assumption will be reasonably correctsince the screens are usually maintained concentric by means of thinrings of low-loss insulating material.

The total current I is equal to the sum of two components contributedrespectively by the two sectors, or

Substituting from equations (2) and (3) it follows that Z 1/27ryEquation 5 shows that the two screens act like a generator whoseterminals are 10 and 11 (Fig. 1), whose electromctive force is and whoseinternal impedance is l/Y, where Yz21ry, the total admittance betweenthe screens. In general for a well designed transformer, the conductancecomponent of Y will be negligible compared with the susceptancecomponent, so that Y y'wK approximately where K is the total capacitancebetween the two screens. It will be assumed in what follows that theconductance components of the transformer admittances can be neglected.

Although actual transformers will only be approximately in accordancewith the assumptions which have been made, generally only the magnitudeof the effect will differ somewhat from the magnitude indicated by theabove formulae, but the character of the effect will be the same. Evenwhen the cross-section of the core and screens is rectangular or squareinstead of circular (as with a shell type core) the electromotive forcee and its effective internal impedance will not be greatly differentfrom the values indicated by the formulae.

Fig. 6 shows a schematic circuit of the transformer in order to indicatehow the electromotive force e acts between the points 2| and 22 of thescreens through the interscreen capacitance K. The external terminals inand II are connected respectively to the points 21 and 22 by conductorswhich can be assumed not to have any appreciable electromotive forcesinduced therein.

Now that the principal cause of the dynamic capacitance effect has beenfully explained, it is possible to state that the present inventionconsists in providing a double screened transformer with means forreducing or eliminating the undesired coupling between the input andoutput sides, which undesired coupling results from electromotive forcesinduced in the screens by alternating currents in the transformerwindmgs.

It will be noted that a, c and (p in Equation 6 can be chosen so that e'is zero. This affords one of the methods according to the invention foreliminating the effect of e. If the tapping points 2| and 22 where theleads to the terminals HI and II are connected are chosen so that 21r+((u+fi) :0, then the undesired effect will be substantially removed. Theinterpretation of this condition is that the points 2| and 22 should beseparated by an angle (p equal to the angle between the two gaps, butthat 2| should be nearer to UP than 22 as measured circumferentially ina clockwise direction from P; in other words the inner screen point Z|should be nearer the outer screen gap I1 than the outer screen point 22.Special cases of these requirements are (a) when (p=1r, or the two gapsare diametrically opposite to one another, then the tapping pointsshould be diametrically opposite to one another; and (b) when 7r=0,(that is the gaps are exactly aligned) in which case the tapping pointsshould be exactly aligned in order to fulfill the condition cz+fl=21n Itis worthy of note that the worst condition which will produce thelargest dynamic capacitance is when on, c and (p are all zero. Theeffective electromotive force e then has the maximum value E.

It should be pointed out with reference to Fig. that if the two overlapsare exactly aligned, the angle (7) will not be zero, but will have avalue equal to the angle subtended by the overlap. This can be seen inFig. 7. If it is necessary for mechanical reasons that the two points 2|and 22 should be exactly aligned, then either one of the screens shouldbe rotated slightly so that the radii 6F and [IQ coincide, which meansthat the two overlaps lie in immediately adjacent sectors, or as shownin Fig. 8, one of the overlaps could be made in the opposite sense, inwhich case BP and 0Q would coincide with the overlaps in the samesector.

It may be pointed out that when one terminal of the primary winding I isconnected to the outer screen 5 (as is often the case) then it ispossible to balance out the effect of the electromotive force e actingthrough the capacitance K against the effect of the input voltage Ebapplied to the primary winding I, which acts through the staticcapacitance 0 effective between the other terminal of the primarywinding and the screen 3. This can be understood from Fig. 9, whichshows the equivalent circuit of this arrangement. Terminal 1 of theprimary winding is supposed to have been connected to the screen 5, andit will be seen by reference to Fig. 6 that E0 in. series with thestatic capacitance 0 comes effectively in parallel with e in series withthe inter-screen capacity K, between the terminals l0 and II, as shownin Fig. 9A. The arrangement is identically equivalent to Fig. 9B inwhich V: Eoc-l-eK It is possible to arrange the tapping points 2| and 22of Fig. 5 so that or Eo/6=K/C. However, as it may be found difiicult inpractice to carry out this adjustment with sufficient accuracy, analternative would be to make an approximate adjustment so that e is alittle too small, and to connect a small trimming condenser between thescreens 3 and 4 which can be adjusted to make up the value of K so thatit satisfies condition (6). This condenser need not actually beinstalled in the confined space between these screens, but could beplaced in and connected to the outer screen 5 as indicated in Fig. 10 at23. The condenser 23 should, however, be in a separate compartment so asto be screened from the winding I, otherwise the static capacity 0 willbe increased. The condenser should be connected to the screen 3 by alead having a screen connected to the outer screen 4.

A slightly different method of satisfying Equation 6 is based on thefact that K/c is generally much greater than Eu/e, so that instead ofadjusting the tapping points 2| and 22 so that e is reduced, a smalltrimming condenser (not shown) could be connected between terminals 6and N). This trimming condenser then comes in parallel with the staticcapacitance c and could be adjusted effectively to increase 0 untilequation (6) is satisfied. However, it is generally better to reduce eas much as possible by the methods already explained before applying anysuch external compensation means. It should also be pointed out that itmay not always be easy to ensure that E0 and e are in exact phaseopposition.

The method of arranging the tapping points and overlaps which has beendescribed for the purpose of reducing the spurious electromotive force 6substantially to zero is thus the preferred method of dealing with thedynamic capacitance effect according to the invention. Other meansexternal to the transformer winding assembly will be presentlydescribed, and these means may be applied additionally to the adjustmentof the tapping points and screen overlaps for dealing with any residualeffect which may remain after such adjustments have been made.

It may, of course, be found impracticable to make the above describedspecial adjustments on the transformer winding assembly, in which casethe external means to be described may be employed instead of suchspecial adjustments. Thus, according to another feature of theinvention, instead of reducing e to zero, or instead of making thespecial adjustments described with reference to Fig. 10 (which latterarrangements are not permissible unless one terminal of the primarywinding is connected to the screen 5), the effect of e may becompensated in the manner indicated in Figs. 11 and 12.

In Fig. 11, a compensating windingyZt of a few turns is inductivelycoupled with the windings and 2 and is connected in series with atrimming condenser 25 between the screens 3 and 4. The circuitarrangement is then as shown in Fig. 9 (C) which will be seen to beessentially similar to Fig. 9 (A) with the winding I replaced by thecompensating winding and the static capaci tance 0 replaced by thetrimming condenser 25. It will be evident from the discussion of formula(6) above that provided the compensating winding is poled so that theelectromotive force 'eo generated thereby is in opposition to the screenelectromotive force 6, then the trimmer condenser '25 can be adjusted sothat the equivalent electromotive force V (Fig. 9B) is zero. It may beadded that with this arrangement the necessary phase opposition between80 and e is likely to be substantially exactly obtained owing to theby-passing of the leakage inductance of the transformer.

As it will be impracticable to place the elements 24 and 25 between. thescreens, they may be arranged as shown in Fig. 12. A screened conductoris employed for the winding 2s and is wound on top of the primarywinding I, care being taken to insulate the conductor screen from thewinding I and also to'insulate it so that it does not produce anyshort-circuited turns. The condenser 25 is housed in aseparatecompartment of the outer portion of the screen l so as to be screenedfrom the winding 1, as described with reference to Fig. 1%. In Fig. 12the winding 24 is shown with only one turn in order to avoid confusingthe figure, but it is to be understood that there may be any suitablenumber of turns.

Fig. 13 shows another very satisfactory method of neutralising thescreen electromotive force 2 by the use of two double screenedtransformers which are connected so that the screen electromotive forceof one is in opposition. to and neutralises that of'the other. The twotransformers shown are each the same as shown in 1 and their parts havebeen given the same designation numbers distinguished by the letters Aand B. They should be of exactly similar design and manufacture andshould be selected by tests so that the values of the screenelectromotive force of the inter-screen capacitance K are as nearly aspossible the same for both. The two pr. .ary windings are then connectedin parallel by suitable leads so as to maintain the screenin but shouldbe connected in opposition, that is terminal EA should be connected toEB, and 613 to 5A. It then 6A and EA are used as the input terminals,the screen electromotive forces e will be of opposite sign becauseopposite directions in the two primary windings. If the terminals NBAand 16B of the inner screens and 55B are connected together then theresultant screen. electromotive force will be substantially zero.

The two secondary windings are preferably connected in series opposing,for example by connecting terminals 8A and 8B and using 9A andBB outputterminals. The secondary windings could-also be connected in parallelopposing if desired, but it can be shown that the effect of any slightresidual screen electromotive force is minimised by increasing thenumber of turns of the secondary winding. This is generally true of anyof the transformers which have been discussed: the effect of a givenvalue of e can always be reduced by increasing the number of turns onthe secondary winding.

In Fig. 13 the primary windings could have been connected in seriesopposing if desired. This arrangement gives excellent compensation overa wide frequency range, but has the disadvantage of doubling theinter-screen capacitance K, and also the bulk and cost. Thus the cost ofthe arrangement likely to be considerably greater than any of the otherschemes which have been described, but the final result is likely to beon the whole better.

The methods according to the invention which have been described so farare applicable to a the current flows in 10 double screened transformerirrespective of the particular use to which it will be put. Anothermethod which will now be described is useful where double screenedtransformers are employed in impedance bridge circuits.

In non-symmetrical bridge networks (that is, those with unequal ratioarms, or those of the so called product arm type) it is not generallydesirable to allow the system of screens to impose a significantcapacitance across more than one arm of the bridge, though inter-screencapacitances which fall across either of the diagonals can bedisregarded, except when considering questions of bridge sensitivity.This restriction involves the use of either triple screened transformers(which would usually be impracticable owing to their complexity), or ofpairs of double screened transformers connected in tandem.

shows one example of a screened impedance bridge of known type with twotandem- .conne'cted input transformers. The four corhers of the bridgeare lettered A, B, C and D. AB and BC contain the ratio arms Z1 and Z2.CD includes the terminals for the impedance Z4 to be measured, and Z3 isthe adjustable impedance .used to balance the unknown impedance. Twoinput transformers 2G and 21 are connected in tandem and are of theconventional double screened type. The secondary winding of 21 isconnected between A and C, the primary winding of 21 is connected to thesecondary winding of 26, and the primary winding of 26 is connected tothe input terminals 28 and 29 to which the test oscillator (not shown)is intended to be connected. The output terminals 36 and 3f for thedetector (not shown) are connected to B and D, respectively.

The elements of the bridge are provided with a system of three screens,the outermost of which is shown in double weight full lines and isconnected to the corner D and earth. The outer screen of the transformer26 is a D screen.

The innermost screen surrounds the leads connected to the corner C andis connected to A.

The inner screen of the transformer 27 is also an A. screen. This screenis shown in dashed lines.

Between the D and A screens is the intermediate screen which surroundsthe impedances Z1 and Z2, the leads connected to the corner A, and the Cscreen. This intermediate screen is shown in. single weight full lines,and is connected to B. It includes the inner screen of transformer 25and the outer screen of transformer 21. Both windings of bothtransformers have one end connected to the correspondin screen. It willbe seen that with this arrangement the capacitance between the A and Bscreens shunts the arm AB and that between the B and D screens shuntsthe arm BD, and there is no screen capacitance shunting any other arm ofthe bridge.

Fig. 15 shows a simplified equivalent circuit of the bridge showing howthe screen electromotive forces of the transformers 25 and 21 act uponthe bridge network. In this figure e1 and Y1 are the screenelectrcmotive :force and interscreen admittance of the transformer 25,and er and Y2 are the same quantities for the transformer 2?. t will beevident that the errors produced by 61 and c2 cannot be easily allowedfor; in fact any attempt to apply corrections would be impracticable.

However, the effect of ca for the transformer 21 can be dealt with byany of the methods already described, and c2 can be reducedsubstantially to zero. The eifect of in for the transformer 26 1 I couldalso be dealt with in a similar way, but in this case an alternativescheme is possible as shown in Figs. 16 and 17.

Fig. 16 shows a modification to the connections of the two transformers26 and 27 of Fig. 14, the rest of the figure being unaltered. The innerwinding of 26 and the outer Winding of 21 are each disconnected from thecorresponding screen and are connected together by a pair of wiresinside the B screen. A difierential condenser 32 is arranged inside theB screen with the fixed plates connected respectively to the abovementioned connecting wires, and the movable plate connected to D. Fig.1'7 shows a schematic circuit of the connection between the two coils,including the screen electromotive force c1 and screen admittance Y1 ofthe transformer 26. The circuit is seen to be a bridge in which 31 and.92 indicate the effective capacitances between the terminals 6, 3 and7, 9 respectively, and the B screen, and v1 and 222 the capacitances ofthe differential condenser 22. It is assumed that the admittance Y1 canbe taken to be substantially a capacitance K1.

It can easily be shown by applying Kirchhofis laws to the bridge networkthat the difference of potential between the B and D corners of thebridge of Fig. 17 will be zero if in which ,u1=61/E1 for the transformer26. It will be evident that provided the condenser 32 has a suitablerange, it can be adjusted to satisfy the above equation and so thedifference of potential between B and D resulting from e1 is balancedout.

It should be pointed out that the above assumes that e1 and E1 are inthe same or in opposite phase, and that $1 and .92 are purecapacitances. Both of these conditions are only approximately fulfilled,but it is found that the arrangement generally produces a considerablereduction of the errors caused by e1. It will be understood that thearrangement does not affect the electromotive force e2 for transformer27 which must be dealt with by one of the methods previously described.

What is claimed is:

1. A double screened electrical transformer comprising a substantiallycylindrical core, a secondary winding on the said core, a substantiallycylindrical inner screen surrounding the secondary winding, asusbtantially cylindrical outer screen surrounding the inner screen,each screen having a narrow gap therein running parallel to the axis ofthe core, a primary winding over the outer screen and a connecting leadtapped 01f at a point on each screen, the said tapping points being sochosen that the undesired difiference of the potentials of the saidpoints resulting from the electromotive forces induced to the screens byalternating currents in the windings is brought within a specifiedlimit, means for deriving from the alternating flux in the core of thetransformer a potential difference acting between the screens to opposethe said undesired difference of potential, and means for adjusting theresultant potential diiference substantially to zero.

2. A transformer according to claim 1 having a small direct capacitancebetween the primary winding and the inner screen, in which one terminalof the primary winding is connected to the outer screen, comprising acondenser connected between the inner and outer screens having such acapacitance that the difference of potential between the screens issubstantially zero.

3. A transformer according to claim 2, in which the condenser is anadjustable condenser.

4. A transformer according to claim 2, in which the condenser isenclosed in the outer screen and is screen-ed from the primary Winding.

5. A transformer according to claim 1 in which cne terminal of theprimary winding is connected to the outer screen, comprising a condenserconnected between the inner screen and the other terminal of the primarywinding and having such a capacitance that the difference of potentialbetween the screens is substantially zero.

6. A transformer according to claim 1 comprising a third winding coupledto the primary and secondary windings and connected in series with acondenser between the inner and outer screens, the capacitance of thecondenser and the numbers of turns and direction of winding of the thirdwinding being so chosen that the potential difference between thescreens is reduced substantially to zero.

7. A transformer according to claim 6 comprising a secondary windingwound over a core, an inner screen surrounding the secondary winding, anouter screen surrounding the inner screen and a primary winding woundover the outer screen, the said third winding consisting of a shieldedconductor is connected to the said outer screen.

WALTER KRAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

